D. D. EDIE the manifold it enters a metering pump. The purpose of this positive-displacement pump is to minimize any pressure fluctuations created by the rotating extruder screw. The metering pump forces the molten precursor into the spin pack. Normally, the spin pack houses a filter, which is capable of Radia Random small solid particles from the molten precursor. As the precursor exits the pack, it is forced through a plate containing numerous small holes (i.e. the spin- neret). Finally, as the molten precursor exits these ing atmosphere and drawn down by the windup device, forming solid fibers. At first glance. this would appear to be a relatively simple process. In fact, the Flat-layer Radial-folded melt spinning is simple. However, melt-spinning mesophase is far from simple Fig 10. Transverse textures of mesophase-pitch-based By applying heat, mass and momentum balane carbon fibres [56] Edie and Dunham [41] showed that the melt-spinning process is extremely sensitiv typical melt-spinning process that might be used to changes in process conditions. Although oduce mesoph did not account for the liquid crystalline behavior of shown in Fig 8. The precursor (in this case, meso- the mesophase precursor, it nevertheless demon- hase pitch) is loaded into the feed hopper of the strated that, at typical process conditions, the tensile xtruder as solid chips. The extruder's rotating screw stress on mesophase fibers is ca 20% of that required to the melting secti to break fiber (see Fig 9), and extruder where the chips are heated, forming a vis- measurements confirm these predictions In compari cous melt. Then the molten precursor is conveyed son, during melt spinning the tensile stress developed into the pumping section of the extruder. In this within a nylon fiber is 1% of the breaking strength section of the extruder the channel narrows, increas- of the filament. This stability problem is the direct the fluid pressure, The molten w at result of two peculiarities of mesophase: its highly a relatively high pressure, exits the extruder and flows temperature-dependent viscosity. and the brittle through the transfer manifold As the precursor exits nature of as-spun mesophase fibers PAN-based fibers Current PAN-based fibers (Prior to 1990) Akzo Nobel, Mitsubish Current mesophase-pitch-based fibe Nippon Steel)(Amoco, Mitsubishi) Isotropic-pitch-based fibers Mesophase-pitch-based fibers (Textron) Prior to 1990) Fiber modulus, GPa Fig. 11. Mechanical properties of commercial PAN-based and mesophase pitch-based carbon fibers as of 1989 compared to the properties of current commercial PAN-based and pitch-based fibers352 D. D. EDK Radial Onion-skin Random Flat-layer Radial-folded Line-origin the manifold it enters a metering pump. The purpose of this positive-displacement pump is to minimize any pressure fluctuations created by the rotating extruder screw. The metering pump forces the molten precursor into the spin pack. Normally, the spin pack houses a filter, which is capable of removing any small solid particles from the molten precursor. As the precursor exits the pack, it is forced through a plate containing numerous small holes (i.e. the spin￾neret). Finally, as the molten precursor exits these holes, it is simultaneously quenched by the surround￾ing atmosphere and drawn down by the windup device, forming solid fibers. At first glance, this would appear to be a relatively simple process. In fact, the melt spinning is simple. However, melt-spinning mesophase is far from simple. nature of as-spun mesophase fibers. By applying heat, mass and momentum balances, Edie and Dunham [41] showed that the mesophase melt-spinning process is extremely sensitive to small changes in process conditions. Although their model did not account for the liquid crystalline behavior of the mesophase precursor, it nevertheless demon￾strated that, at typical process conditions, the tensile stress on mesophase fibers is ca 20% of that required to break the fiber (see Fig. 9), and experimental measurements confirm these predictions. In compari￾son, during melt spinning the tensile stress developed within a nylon fiber is < 1% of the breaking strength of the filament. This stability problem is the direct result of two peculiarities of mesophase: its highly temperature-dependent viscosity, and the brittle Fig. 10. Transverse textures of mcsophasc-pitch-based carbon fibres [56]. typical melt-spinning process that might be used to produce mesophase pitch precursor fibers [40] is shown in Fig. 8. The precursor (in this case, meso￾phase pitch) is loaded into the feed hopper of the extruder as solid chips. The extruder’s rotating screw conveys the chips into the melting section of the extruder where the chips are heated, forming a vis￾cous melt. Then the molten precursor is conveyed into the pumping section of the extruder. In this section of the extruder the channel narrows, increas￾ing the fluid pressure. The molten precursor, now at a relatively high pressure, exits the extruder and flows through the transfer manifold. As the precursor exits 7 6 5 4 3 2 1 0 C I I I I I I I I I PAN-based fibers Current PAN-based fibers (Prior (Amoco, Hercules, Akzo Nobel, Mitsubishi) Isotropic-pitch-based fibers Mesophase-pitch-based fibers l v (Textron) (Prior to 1YYO) I I I I I , I I I ) 100 200 300 400 500 600 700 800 900 1 Fiber Modulus, GPa 10 Fig. 11. Mechanical properties of commercial PAN-based and mesophase pitch-based carbon fibers as of 1989 compared to the properties of current commercial PAN-based and pitch-based fibers